Dental alloys for porcelain-fused-to-metal restorations

ABSTRACT

A dental alloy for use in porcelain-fused-to-metal restorations including palladium, cobalt, gallium, gold, aluminum, copper, zinc and ruthenium or rhenium. The cobalt controls the coefficient of thermal expansion of the alloy to permit the use of the alloy with commercially available porcelains having a variety of thermal coefficients. The zinc serves as a scavenger during formation and casting of the alloy. The aluminum protects the alloy from absorbing gases during torch melting and during the porcelain firing process. The ruthenium or rhenium provides grain refining for the alloy to increase its elongation, tensile strength, and thus toughness. The alloy with ruthenium or rhenium as a grain refining agent must be made in a protective environment to avoid the formation of bubbles in the porcelain during the porcelain firing process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 528,227 filedAug. 31, 1983 which application was a continuation-in-part ofapplication Ser. No. 400,481 filed July 21, 1982 now U.S. Pat. No.4,419,325.

BACKGROUND OF THE INVENTION

This invention relates to palladium based dental alloys and, inparticular, to alloys for use in porcelain-fused-to-metal restorations.

Porcelain-fused-to-metal restorations consist of a metallicsub-structure coated with a veneer of porcelain. Over the years variousalloys have been proposed for the sub-structure of these restorations.Many of the early alloys used gold with some platinum or palladium asthe main alloy ingredients. However, with the increases and fluctuationsin the price of gold and platinum in recent years, other alloys havecome to play major roles in this area. One series of alloys which hasgained general acceptance is based on nickel, chromium and beryllium andthe main ingredients. Another series of alloys, with which thisinvention is concerned, is based on palladium as the dominant element.

One such palladium based alloy for use in porcelain-fused-to-metalrestorations is described in U.S. Pat. No. 4,261,744. This alloyincludes approximately 80% palladium and lesser amounts of indium, tin,cobalt and silicon. Another palladium alloy, which was commerciallyavailable prior to this invention, includes, based on spectrographgicanalysis, approximately 2% gold, 79% palladium, 9% gallium, 10% copperand perhaps a trace of boron (on the order of 0.1%). Alloys similar tothis commercial alloy, which include gold, palladium, gallium, copperand boron are described in U.S. Pat. Nos. 3,134,671 and 4,179,288.

In examining the commercially available gold-palladium alloy describedabove, it was found that the alloy suffered a number of disadvantages interms of its suitability for use in porcelain-fused-to-metalrestorations. In particular, the alloy exhibited poor grain structurewhich gave it low elongation, lower than optimum tensile strength andlow toughness, as well as making it susceptible to "hot-tearing" duringthe investment casting process.

Surprisingly, in seeking to overcome these limitations, numerousdifficulties were encountered in attempting to grain refine this alloy.In particular, it was found that when the standard grain refiningtechniques were applied to the alloy, and the alloy then used to make acasting for a porcelain-fused-to-metal restoration, the casting causedbubbles to form in the porcelain during the porcelain firing process.This resulted in an unusable restoration.

Moreover it was found that the commercial gold-palladium alloy had acoefficient of thermal expansion which was not compatible with the fullrange of porcelains available for porcelain-fused-to-metal restorations.In particular, although the alloy could be used with porcelains having alow coefficient of thermal expansion, it could not be used withprocelains having a high coefficient, particularly for long-spanbridgework involving pontics.

Accordingly, it is one of the objects of this invention to overcome thelimitations of the above described commercially available palladiumbased dental alloy. In particular, it is an object of this invention toprovide a grain refined palladium based dental alloy which will notproduce bubbles when porcelain is applied. It is a further object of theinvention to produce a palladium based dental alloy which has acoefficient of expansion compatible with the complete range of dentalporcelains commonly used in porcelain-fused-to-metal restorations.

The attainment of these and other objects of the invention is describedbelow in connection with the description of the preferred embodiments ofthe invention.

SUMMARY OF THE INVENTION

In accordance with the invention, a palladium based dental alloy isprovided which consists essentially of approximately 35-85% by weightpalladium, 0-12% by weight copper, 5-15% by weight gallium, 0-50% byweight gold, 0-5% by weight aluminum, 0-13% cobalt, 0-0.5% zinc and0.1-0.5% ruthenium, rhenium or mixtures thereof, the total of theconstituents being 100%. Preferred embodiments of the alloy haveapproximate compositions by weight as follows:

    ______________________________________                                        Pd     Cu     Ga      Au  Al    Co    Zn  Ru or Re                            ______________________________________                                        78.7   10     9       2.0 0.1    --   --  0.2                                 78.7    7.5   9       2.0 0.1    2.5  --  0.2                                 80.7   --     9       --  0.1   10    --  0.2                                 78.6   10     9       2.0 0.1   --    0.1 0.2                                 78.6    7.5   9       2.0 0.1    2.5  0.1 0.2                                 80.6   --     9       --  0.1   10    0.1 0.2                                 ______________________________________                                    

The ruthenium or rhenium in these alloys serves as a grain refiningagent. In accordance with the invention, to introduce these agents, thealloy must be made in a protective environment, such as, under vacuum orin a reducing or an inert atmosphere, e.g., at atmosphere of argon. Ifnot done in this way, the alloy that is produced will contain absorbedgases which will cause bubbling of the porcelain during the porcelainfiring process. Importantly, iridium, which is a known grain refiningagent, is excluded from the invention because it fails to grain refinethe alloy.

In copending application Ser. No. 400,481 it was shown that the use of aprotective environment during the formation of the alloy and theincorporation of aluminum in amounts up to about 5% as part of the alloyessentially eliminate bubble formation during the porcelain firingprocess. For most conditions, a level of aluminum on the order of 0.1%has been found sufficient to eliminate bubbling. This is a desirablelevel for aluminum since it results in alloys which melt like preciousalloys and generally leave a clean, i.e., metal-free, crucible when themolten alloy is cast. Under some conditions, however, e.g., overheatingof the alloy during the casting process, the 0.1% level for aluminum hasbeen found to be insufficient to eliminate completely bubble formationduring the porcelain firing process. Although higher levels of aluminumcan be used to guarantee bubble-free restorations, even for overheatedalloys and the like, such higher levels result in an alloy which meltslike a non-precious alloy and which leaves a film of metal and slag inceramic crucibles. As a general proposition, dental laboratories preferalloys whose melting characteristics are similar to gold, rather than tonon-precious alloys. Also, the leaving behind of a film of metalrepresents a loss of material and thus is undesirable from an economicpoint of view.

It has now been found that bubble-free restorations can be achieved withlow levels of aluminum through the inclusion of small amounts of zinc inthe alloy. Such zinc-containing alloys melt like precious alloys andleave an essentially clean crucible. Moreover, these alloys have beenfound to produce finished restorations which are essentially bubble-freefor a wide range of processing conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate diagrammatically the importance of the relativecoefficients of thermal expansion of the alloy and the porcelain. InFIG. 1, the coefficient of expansion of the alloy is greater than thatof the porcelain so that the porcelain is under longitudinal compressionin the final fused product, as is desired. In contrast, FIG. 2illustrates the undesirable situation where the porcelain is underlongitudinal tension in the final fused product because the coefficientof thermal expansion of the alloy is less than the coefficient ofthermal expansion of the porcelain. The changes in length shown in thesefigures are for purposes of illustration only, and are not to scale.

FIG. 3 is a plot of thermal expansion (K_(T)) versus temperature forthree alloys having 10% cobalt (the upper curve), 2.5% cobalt (themiddle curve) and no cobalt (the lower curve). The remainder of thecomposition of these alloys is given below in Table I.

FIG. 4 is a photomicrograph showing the grain structure of thecommercially availablee gold-palladium alloy discussed above.

FIGS. 5 and 6 are photomicrographs showing the improved grain structureof the alloys of this invention when ruthenium (FIG. 5) or rhenium (FIG.6) are used as grain refining agents.

FIG. 7 is a photomicrograph showing the poor grain structure of thealloy when iridium is used as the grain refining agent.

FIG. 8 is a photograph of the porcelain surface produced when thegrain-refined alloy is made in an inert atmosphere.

FIG. 9 is a photograph of the porcelain surface produced when thegrain-refined alloy is made in air.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The alloys of this invention can include the following constituents:gold, palladium, gallium, copper, aluminum, cobalt, zinc, ruthenium andrhenium.

Palladium and gold give the alloy its basic inertness so that it canwithstand the environment of the patient's mouth. The relative amountsof these two components can be varied without changing the properties ofthe alloy. The palladium concentration of the alloy is preferablybetween about 35 and 85 wt. %, and most preferably between about 70 and85 wt. %. The gold concentration is preferably between about 0 and 50wt. %, and most preferably between about 0 and 10 wt. %.

Gallium and cobalt/copper reduce the melting point and strengthen thealloys. These components also form a protective and adherent oxide onthe surface of the casting which reacts with the porcelain to produce achemical bond. Of these elements, it was found that the combination ofgallium and cobalt produces an oxide which is better for bondingporcelain than the oxide formed from the combination of gallium andcopper.

The gallium concentration is preferably between about 5 and about 15 wt.%, and most preferably between about 6 and about 10 wt. %. The cobaltconcentration is preferably between about 0 and 13 wt. %. The copperconcentration is preferably between about 0 and 12 wt. %. When cobalt isused without copper, the cobalt range is most preferably from about 4 toabout 10%. When copper is used without cobalt, the copper range is mostpreferably from about 5 to about 10%. If both copper and cobalt areused, the copper range is most preferably from about 5 to about 10%, andthe cobalt range is most preferably from about 2 to about 6%.

As discussed in detail below, the cobalt serves to control thecoefficient of thermal expansion of the alloy. The amount of thiscomponent is adjusted, for example, by substituting cobalt for copper,to provide coefficients of thermal expansion compatible with thecomplete range of porcelains available for porcelain-fused-to-metalrestorations.

The aluminum serves to protect the alloy during torch melting and alsoduring the porcelain firing process. Specifically, as the alloy is torchmelted prior to being cast, the aluminum forms an oxide on the outsideof the metal. This oxide substantially reduces the absorption of gasesby the molten alloy. Such gases, if permitted to be absorbed, couldlater be released during the porcelain application process and thus formbubbles in the porcelain. Similarly, during the porcelain firingprocess, the aluminum forms a protective oxide when the metalsubstructure is heated.

Aluminum can be used in the alloy in amounts ranging up to about 5 wt.%. The preferred range for aluminum is between about 0.05 and 2 wt. %,with the most preferred concentration being approximately 0.1 wt. %.Higher amounts of aluminum can be used in place of gallium to lower themelting point and to strengthen the alloy.

The inclusion of zinc in the alloy serves to further reduce bubbleformation during the porcelain firing process. The zinc functions as ascavenger during formation of the alloy and during the casting process.It has been found that small amounts of zinc, in combination with aprotective environment and the use of aluminum, protect the alloy duringmanufacture, torch melting and the porcelain firing process, resultingin essentially complete elimination of bubbles in the finishedrestorations. Preferably between about 0.1 and 0.5 wt. % of zinc isincluded in the alloy, and most preferably between about 0.1 and 0.25wt. %.

As discussed above, inclusion of zinc in the alloy allows for the use oflower levels of aluminum, e.g., on the order of about 0.1%, so as toproduce an alloy which (1) melts like a precious alloy, (2) leaves ametal-free crucible during casting and (3) produces finishedrestorations which are essentially bubble-free for a wide range ofprocessing conditions. The amount of zinc must be controlled in view ofthe presence of gallium in the alloy. In particular, zinc cannot be usedin large quantities (e.g., more than 0.5% wt. %) with gallium because ofthe formation of a low melting phase along grain boundaries which makethe alloy susceptible to tearing or fracture. Silicon, magnesium ormixtures thereof can be used to replace all or part of the zinc in thealloy. Of these three elements, zinc is considered the most preferred.When silicon is used in the alloy, its concentration is preferably keptbelow about 0.25%; when magnesium is used in the alloy, itsconcentration is preferably kept below about 0.50%.

Cobalt is used in the alloy to provide flexibility in the adjustment ofthe alloy's coefficient of thermal expansion. Gallium, copper andaluminum also affect the coefficient of thermal expansion, but to a muchlesser extent. Flexibility in the ability to adjust the coefficient ofthermal expansion is necessary in view of the broad range of porcelainsavailable in the market.

FIGS. 1 and 2 illustrate diagrammatically the effect on longitudinalcontraction of different relative coefficients of thermal expansion forthe porcelain and the alloy.

In FIG. 1, the metal is assumed to have a coefficient of expansion, andthus a coefficient of contraction, greater than that of the porcelain.Panel A of FIG. 1 shows the porcelain and alloy in their heatedcondition, just after the bond has formed between the porcelain and theoxides on the alloy. Panel B shows the porcelain and alloy, bondedtogether, in their cooled, contracted state. Panel C shows thecontraction that would have occurred in the alloy and the porcelain ifthe two materials had not been bonded together.

Comparing panels B and C, we see that the metal component in panel C hasa length shorter than the bonded porcelain-metal combination, while theporcelain component in panel C has a length greater than the bondedcombination. Accordingly, for the bonded combination, the porcelain isunder compression, because its length is less than the length it wouldhave had if it had not been bonded to the alloy, while the alloy isunder tension, because its length is greater than the length it wouldhave had if it was not bonded to the porcelain.

FIG. 2 shows the identical set of conditions but for the coefficient ofexpansion of the metal being less than that of the porcelain. Againpanel A shows the length of the alloy-porcelain combination in itsheated condition. Panel B shows the length after cooling, and panel Cshows the lengths the individual components would have had if they hadnot been bonded together. In this case, because the metal contracts lessthan the porcelain, the metal is under compression and the porcelain isunder tension.

In terms of porcelain-fused-to-metal restorations, it is important thatthe porcelain be under compression, not tension. If it is under tension,cracks will form in the porcelain to relieve the tension. It is toachieve this condition of porcelain being under compression that varyingamounts of cobalt are used in the alloy of the invention.

The following table illustrates the effect of varying the concentrationof cobalt upon the thermal expansion of the alloy (K_(T)) at 500° C. Thepercentages shown in the first column of this table were determinedusing a Theta differential dilatometer, where the reference temperaturewas 30° C., the rate of temperature climb was 3° C./minute and thereference standard was pure platinum.

                  TABLE I                                                         ______________________________________                                        Alloy  K.sub.T                                                                              Co      Cu  Pd     Ga  Al    Au  Ru                             ______________________________________                                        1      .670   --      10  78.7   9   0.1   2.0 0.2                            2      .685   2.5     7.5 78.7   9   0.1   2.0 0.2                            3      .725   10      --  78.7   9   0.1   2.0 0.2                            ______________________________________                                    

FIG. 3 shows the behavior of K_(T) over the range of temperatures from30° C. to 700° C.

As can be seen from FIG. 3 and Table I, the substitution of cobalt forcopper increases the amount of thermal expansion exhibited by the alloywhich changes in temperature. This allows the production of alloysuseful for a wide range of porcelains, in that, by adjusting the cobaltconcentration, a thermal expansion for the alloy can be obtained whichis greater than the thermal expansion of the porcelain so that, in thefinal restorations, the procelain will be under compression. The thermalbehavior of alloys including small amounts of zinc ranging up to about0.5% are substantially the same as those described above for alloyswithout zinc.

The ruthenium or rhenium component of the alloy provides the importantproperty of grain refining. Alloys consist of individual grains incontact with each other. The size of these grains is critical to thephysical properties of the alloy. This size can vary from coarse tofine, and the grains can be regular or irregular.

Ideally, a dental alloy should have fine, regular grains. Alloys withthis type of grain structure exhibit superior elongation, tensilestrength and toughness properties. Moreover, such alloys are less proneto hot tearing during the investment casting process, as compared toalloys with a coarser grain structure. "Hot tearing", as understood inthe art, involves the formation of cracks in the casting due to stressesproduced in the casting as it cools in the investment. These cracks canresult in failures which necessitate remaking the casting with theconcomitant loss of the time, energy and material used to make theoriginal casting.

In an attempt to improve the grain structure of the alloys of thisinvention ruthenium, rhenium and iridium were tested. Quitesurprisingly, it was found that when these grain refiners were used, andthe alloy was prepared in air, the conventional manufacturing techniquefor precious alloys, the resulting alloy was unsuitable for use in aporcelain-fused-to-metal restoration because it produced bubbles in theporcelain during the porcelain firing process. Only when the alloy wasprepared in a protective environment, was a suitable alloy obtained.Moreover, when the grain refining element iridium was used, only poorgrain refinement was achieved regardless of the particular method ofpreparation employed. This was found to be the case up to and includingiridium concentrations as high as 0.5%.

FIGS. 4, 5, 6 and 7 show the effects of grain refining on the alloys ofthis invention. FIG. 4 is a photomicrograph of the grain structure ofthe commercially available gold-palladium alloy described above. As canbe seen, the grain structure is coarse.

FIGS. 5 and 6 show the alloys of this invention where 0.2% by weightruthenium or rhenium, respectively, have been added. The FIG. 5 alloyhas the composition of alloy 1 in Table I; the FIG. 6 alloy has the samecomposition but with rhenium in place of ruthenium. As can be seen fromthese photomicrographs, the grain structure is now significantlyimproved in comparison to the commercially available alloy, and thealloy consists of regular, small grains. Essentially the same grainstructure is achieved when the alloy includes small amounts of zinc.

FIG. 7 shows the situation when 0.2% iridium is used as a grain refiner.This alloy has the same composition as the alloys of FIGS. 5 and 6 butwith iridium substituted for ruthenium or rhenium. Plainly, only verypoor grain refining has been achieved and the grain of this alloy ismore similar to that of the commercially available alloy (FIG. 4) thanthat achieved with ruthenium or rhenium (FIGS. 5 and 6). This poor grainstructure still results even if the alloy includes zinc.

Table II shows the effect of grain refining on the physicalcharacteristics of the alloy. Alloy A in this table has the compositionof alloy 1 in Table I; alloy B has the same composition but with 0.2%more palladium and no ruthenium. As shown in the table, grain refiningproduces an alloy having increased strength, increased elongation andthus increased toughness. An Instron machine was used to measure thevalues reported. The same improved physical properties were observedwhen rhenium was used as the grain refining agent, but not when iridiumwas used. Also, the improved physical properties were found in thepresence of zinc.

                  TABLE II                                                        ______________________________________                                                            Ultimate                                                  Alloy Yield Strength                                                                              Tensile Strength                                                                           Elongation                                   ______________________________________                                        A     150,000 psi   175,000 psi  12%                                          B     130,000 psi   151,000 psi   9%                                          ______________________________________                                    

As mentioned above, the standard technique for forming a grain-refinedalloy cannot be employed with the alloys of this invention because itleads to the formation of bubbles in the porcelain during the porcelainfiring process. Rather, the grain-refined alloy must be formed in aprotective environment, such as, under vacuum, in a reducing atmosphereor in an inert atmosphere, for example, an atmosphere of argon. Withoutproceeding in this way, the alloy absorbs gases from the atmospherewhich are later released from the alloy during firing to form bubbles inthe porcelain. Also, it has been found that carbon containing cruciblesare not advantageous in the preparation of the alloys of the presentinvention. Rather, ceramic crucibles, e.g., zirconia crucibles, arepreferred.

FIGS. 8 and 9 illustrate the difference between forming the alloy in airand under the conditions of this invention. In each case, the alloy hasthe composition of alloy 1 in Table I.

FIG. 8 shows the surface of the porcelain when the elements making upthe alloy including the grain refining agent are combined under ablanket of an inert gas, such as argon. The argon is preferrablyintroduced after vacuum has been applied to the melting chamber toremove ambient air. Alternatively, a stream of argon can be passedthrough the chamber without first drawing a vaccum. As can be seen inFIG. 8, the procelain is smooth and bubble free. The same smoothporcelain surface also is achieved when the constituents are combined ina reducing atmosphere or under vacuum. When only a vacuum is used, thetemperature of the melt and the applied vacuum must be controlled inview of the vapor pressures of the components of the alloy to avoidexcessive relative losses of the more volatile components. Inparticular, when zinc is included in the alloy, a protective environmentcomprising a reducing or an inert gas, rather than a vacuum environment,should be used in forming the alloy in view of the relatively high vaporpressure of zinc.

In comparison to the smooth surface achieved when the alloy is made in aprotective environment, FIG. 9 illustrates what happens to the porcelainif the grain-refined alloy is made in air. Plainly, porcelain withbubbles such as those shown in FIG. 9 would not be acceptable.

In addition to the requirement that the grain refined alloy be made in aprotective environment, the grain refining agent must be introducedwithin a specific range of concentrations. In particular, at least 0.1%of grain refining agent must be added to achieve the improved physicalproperties and additions above about 0.5% tend to embrittle the alloy,as well as hardening it to the point where it cannot be rolled withoutfirst being annealed. The preferred range for the grain refining agentis between approximately 0.1 and 0.3 wt. %, the most preferredconcentration being about 0.2 wt. %.

It should be noted that the improved grain and physical propertiesdescribed above result whether the alloy is made in air or in aprotective environment; it is only so that porcelain can later beapplied to a casting made from the alloy that a protective environmenthas to be used in preparing the alloy. Also, the poor grain structureand physical properties described above for iridium result irrespectiveof whether the alloy is made in air, in vacuum or under an inert orreducing atmosphere.

Although specific embodiments of the invention have been described andillustrated, it is to be understood that modifications can be madewithout departing from the invention's spirit and scope. Thus theconcentrations of palladium, gold, gallium, copper, aluminum, zinc,cobalt and ruthenium or rhenium can be varied from the percentagesillustrated and alloys having the superior characteristics of theinvention will still result. For example, the palladium concentrationcan be varied at least between 35 and 85% by weight; the copperconcentration between 0 and 12%, the gallium concentration between 5 and15%, the gold concentration between 0 and 50%; the aluminumconcentration between 0 and 5%; the cobalt concentration between 0-13%;the zinc concentration between 0 and 0.5%; and the ruthenium or rheniumconcentration between 0.1% and 0.5%.

I claim:
 1. A method for reducing bubble formation during the firing ofporcelain onto cast grain-refined palladium based dental alloyscomprising:(a) initially forming a grain-refined palladium based moltenalloy in a ceramic crucible in a protective environment; (b) admixing atleast one oxide-forming element with said molten alloy to protect thealloy from absorbing gases during initial formation of the alloy, torchmelting and casting, and the procelain firing process; and (c) admixingzinc with said molten alloy in an amount sufficient to serve as ascavenger during initial formation of the alloy, torch melting andcasting, and the porcelain firing process; and (d) pouring said moltenalloy into a mold and allowing said alloy to solidify therein in saidprotective environment.
 2. The method of claim 1 wherein the protectiveenvironment comprises an atmosphere of an inert gas.
 3. The method ofclaim 2 wherein the protective environment includes argon.
 4. The methodof claim 1 wherein the ceramic crucible is a zirconia crucible.
 5. Themethod of claim 1 wherein the oxide-forming element is aluminum.
 6. Themethod of claim 5 wherein the concentration of aluminum is between about0.05 and 2.0 percent by weight and the concentration of zinc is betweenabout 0.1 and about 0.5 percent by weight.
 7. The method of claim 6wherein the concentration of aluminum is about 0.1 percent by weight andthe concentration of zinc is between about 0.1 and about 0.25 percent byweight.
 8. The method of claim 1 wherein all or part of the zinc isreplaced by silicon, magnesium or mixtures thereof.